A capacitor bank is frequently used as an energy store, for example in a locomotive or in some other prime-mover rail vehicle, but in particular for stationary use in an overhead line. This capacitor bank includes two or more parallel-connected rows of individual capacitors in each of the rows. These capacitors may be what are referred to as ultracapacitors.
By way of example, the braking energy is temporarily stored in an energy store such as this in order to emit it again when the vehicle drives away. In order to allow as much energy as possible to be stored in a capacitor bank such as this, it is important for all the capacitors in a capacitor bank to be loaded uniformly, that is to say to be charged uniformly and completely. This process is called “balancing”.
If a capacitor voltage which is greater than the nominal voltage is present on individual capacitors, even if only for a short time, then this leads to this capacitor ageing more quickly. In the worst case, it must then be removed prematurely, which is costly. Different voltages on the individual capacitors, which make the energy store inefficient, are essentially caused by capacitor characteristics, for example the equivalent series resistance and/or the resistance of the capacitor. Parallel resistances may also have an effect.
It is thus necessary to charge each individual capacitor in a capacitor bank to an upper voltage limit and to protect it against overcharging, which could lead to destruction of the capacitor. If the voltage that is applied is too high, this could lead to bubbles being formed in the electrolyte, which can burn and is toxic. However, it should be possible through the use of a diagnostic process, to identify in good time whether a capacitor is not fully serviceable.
It has already been proposed for voltage balancing to be carried out for all the capacitors (matching of the voltages which are present on the capacitors and the charges on the capacitors), in order that they are charged uniformly.
One known method provides for each capacitor to be charged individually. Small contactors are required to do this, which connect the capacitors to the power supply successively. This method takes a very long time. Furthermore, complex wiring is required.
It has already been proposed for the individual capacitors all to be balanced immediately on reaching a nominal value, with the aim of achieving uniform charging. A discharge path is required in parallel with each capacitor to do this. This method furthermore takes a very long time, if only a small balancing current is possible.